An object detecting system is described, for example, in "Radar Technology" (issued from Electronic Information Communication Institute, Foundation), and is arranged so that a distance from a vehicle to an object to be detected and a relative speed of the vehicle is obtained from rising-side and falling-side peak frequencies which are obtained by transmitting an FM-CW wave with its frequency increased and decreased into a triangular waveform with time in a forward direction of the vehicle. A wave reflected from the reflecting object existing ahead of the vehicle is received, the transmitted signal and the received signal are mixed with each other to provide a beat signal, and the beat signal is subjected to frequency analysis.
If a plurality of vehicles or roadside objects such as road lamps exist ahead of the subject vehicle, the plurality of vehicles and the roadside objects are simultaneously detected, thereby providing pluralities of rising-side and falling-side peak frequencies. Therefore, if the rising-side and falling-side peak frequencies are not accurately combined, the distance to the object to be detected and the relative speed of the subject vehicle cannot be accurately determined, and a mis-detection may occur.
Therefore, in a system disclosed in Japanese Patent Application Laid-open No. 8-94749, all the rising-side and falling-side peak frequencies are combined, and the distance between the object and the subject vehicle is calculated for all combinations of the rising-side and falling-side peak frequencies. The calculated distance is compared with a predicted distance based on the combinations determined the last time. Thus, the combination of the peak frequencies with the calculated distance nearest to the predicted distance is determined as a combination individually corresponding to the object.
In the system described in Japanese Patent Application Laid-open No. 8-94749, when either one of the rising-side and falling-side peak frequencies forming a pair for the object to be detected is missing due to any cause, a position predicted from the previous combination can be output directly as made-up data, if the period of time for the missing information is short. However, when the missing information is sustained for a long period of time, it is impossible to accurately combine the remaining peak frequencies without the missing information. Hence, the distance between the subject vehicle and the object and the relative speed of the subject vehicle are calculated based on the result of the combination of inappropriate peak frequencies with each other.